A common form of liquid crystal displays includes a pair of transparent substrates between which is located the liquid crystal active layer. On each of such substrates there are included a two dimensional array of picture element electrodes, or pixel electrodes, and a parallel array of linear control electrodes. On one of such substrates it is now common to include a plurality of diode rings, and a separate one of such diode rings is connected between each pixel electrode and one of the control electrodes. The diode rings are useful to improve the performance of the liquid crystal elements.
FIG. 2 shows an equivalent circuit of a conventional active matrix display using diode rings. In the figure, a separate diode ring 22, formed by connecting two diodes serially and further combining two such serial arrangements in parallel with the polarity being reversed, is inserted in series with liquid crystal 21 between each of row control electrodes 31, 32, and column control electrodes 41, 42, for increasing the threshold voltage. The liquid crystal 21 includes on opposite sides the picture element electrodes 21A and 21B. The linear row electrodes are arranged on one of the two substrates, the linear column electrodes on the other, the two arrays being orthogonal to form a two-dimensional rectangular array, and each electrode is associated with a row or column of picture element electrodes. As shown, each diode ring 23 is connected between a pixel electrode 21B and a row control electrode 31 and so would be located on the substrate including the row electrodes 31 and pixel electrodes 21B. The use of two diodes in series in each of the parallel branches doubles the threshold voltage needed to breakdown the diodes.
FIG. 3 shows a liquid crystal matrix device which L has been utilized hitherto, in which FIG. 3(a) is a plan view and FIG. 3(b) is a cross sectional view taken along line A--A thereof. A transparent electroconductive film formed on glass plate 1 is patterned to form pixel electrode 2 and row electrode 3. The row electrode 3 corresponds to the elements 31, 32, and pixel electrode 2 to electrode 21B shown in FIG. 2. Then, a pair of p-i-n diodes 5 made of three layers of amorphous silicon are sandwiched between chromium electrode layers 61, 62 and 63 as shown to constitute diode assembly 50 having two diode stages. The chromium layers are useful for preventing the entrance of light into the diodes, which would generate photocurrent and spurious signals. Accordingly it is necessary to make the thickness of layers 61 and 63 greater than 500 .ANG. in order to obtain reliable light shielding. Moreover, since the thickness of each amorphous silicon diode is generally about 5000 .ANG., the total film thickness of the two stage diode assembly is greater than 1 .mu.m.
Further, after forming over electrode 63 a silicon nitride film 7 in which a contact hole is made, conductor lead 8, of aluminum or the like, is deposited to connect the picture element electrode 2 with the upper electrode 63 of the diode. Another two stage diode assembly 40 of the same kind as that of the above-mentioned two stage diode is simultaneously formed on a portion of the picture element electrode 2 and by connecting its conductor lead 8 to the row electrode 3, a diode ring equivalent with the element 22 shown in FIG. 2 is completed.
There would similarly be prepared a substrate including a linear array of column electrodes and a two-dimensional array of pixel electrodes, without the diodes, and the two substrates are positioned opposite one another, properly oriented. Liquid crystal material is injected therebetween to complete the display device.
Since the height of the two stage diode is greater than 1 .mu.m, as described above, failures such as breaks or short-circuits are apt to result from the steps in either the insulation film 7 or the conductor lead 8. It is possible to deposit the four diodes on a plane in order to prevent such troubles. However, since the area occupied by the diodes is doubled, the ratio of the area occupied by the picture element electrodes to the whole (the aperture ratio) is decreased, resulting in loss of brightness and contrast in the display.